Coating film prepared from fluororesin has a wide range of uses in coating to form fluororesin layers on substrates of articles, which need corrosion resistance, a non-cohesion property and heat resistance, such as bread-baking molds, and rice cookers. However, since the fluororesin is poor in adhesion to substrates made of metals, ceramics or the like because of the non-cohesion property thereof, substrates coated with primers having an affinity for both the fluororesin and the substrate in advance.
The fluororesin layer is generally required to be thickened in uses requiring corrosion resistance. In order to thicken the fluororesin layer, it is necessary to repeat coating of applying a coating composition comprising fluororesin and baking the applied coating composition at temperatures not lower than a melting point of fluororesin. A primer is required to have heat-resistant adhesion capable of withstanding the long-duration baking at elevated temperatures and maintaining adhesion to a substrate and the like.
As the primer excellent in the heat-resistant adhesion, there has been widely adopted a primer based on chromate phosphate, having excellent resistance to long-duration baking at elevated temperature, until today. However, since there is growing awareness of environmental issues, the development of a chromium-free primer, which does not contain hexavalent chromium but has the strong heat-resistant adhesion comparable to the primer based on chromate phosphate, has been strongly desired over the years.
As a chromium-free primer, a combination of fluororesins and various binder resins has been conventionally studied. As the binder resins, there was proposed the use of polyphenylene sulfide (PPS) from the viewpoint of heat resistance. However, PPS had a problem in that PPS was poor in compatibility with the fluororesin and adhesion to the fluororesin was insufficient.
In order to improve the adhesion to the fluororesin, it was proposed that polyamide-imide (PAI) and/or polyimide (PI) are/is added to PPS as the binder resin in the chromium-free primer (see, for example, Japanese Kokai Publication Sho-53-74532), In an example in this publication, PAI and PPS are used in a ratio of 1:15 to 1:20.
As the chromium-free primer using PPS and PAI as binder resin, one using PAI and PPS in a ratio of 3:1 to 1:3 was also proposed (see, for example, U.S. Pat. No. 5,789,083). However, this chromium-free primer has a feature in blending two kinds of fluororesins differing in melt viscosity each other in a specific ratio in order to provide a water-based primer capable of applying to a smooth surface, and there was a problem in that heat-resistant adhesion was deteriorated due to a long-duration baking.
As the binder resin of the chromium-free primer, one having PAI and PPS in a ratio of 1:1 is known (see, for example, Japanese Kokai Publication Hei-8-322732), but there was a problem in durability for hot water.
Thus, the binder resin comprising PAI and PPS has been conventionally developed on a course of adding a small amount of PAI to a large amount of PPS.
As a composition comprising PAI and an antioxidant, for example, PAI is exemplified as a resin (I) having at least one of an ester bond, an amide bond and an imide bond, and there is disclosed a substance comprising the resin (I) and an antioxidant (III) which accounts for 0.1 to 5 weight % of the resin (I) (see, for example, Japanese Kokai Publication Hei-2-4880). However, this composition is limited to use in coating of copper such as copper wires and there are no descriptions or suggestions about uses as a primer. In addition, there is no description of the use of poly(arylene sulfide) such as PPS as the antioxidant (III).
Although PAI is employed in various parts where heat resistance is required after a process of kneading and molding, there has been a problem in that amide groups of PAI were low in heat resistance and PAI is likely to deteriorate at elevated temperature.